Measuring TIR, which measurement do I believe?

WobblyHand

WobblyHand

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Been having some issues with my drill press. Either bent drill bits or something else. Thought it would be good to measure the TIR. Used a ground 3/8" rod mounted in my drill press chuck, using a DTI and a dial indicator. DTI was a no name thing. It seemed to indicate a lot of runout. 0.003" or so. I then used an old Federal dial (plunger) indicator (0.0001" per division), it measured 0.0012" TIR (I think, but see below). Both held by my Noga holder. Made sure neither of them were bottomed out. The DTI has a little ball, whereas the old Federal has a relatively large hemispherical contact point.

Contact point was 1.5" down from chuck. Like to believe the Federal. Have an old Enco (made in Japan) DTI that I can try. Since I'm just a newbie, I'm well aware that technique (or lack of it) can change the answer. Like when I forgot to tighten all three points on the Jacobs chuck - that reduced the TIR from 0.005" to 0.003 on the no name DTI.

Perhaps I don't quite know how to use the DTI. It was relatively flat to reduce cosine errors. How flat does it have to be? For measuring a rod, does it have to be 90 degrees to the axis of the workpiece and the lever tangential to the surface?

When using the Federal dial indicator I noticed the run out didn't strictly repeat. By that I mean the low point never got that low again. The high point always seemed to repeat. So say it started at 0, then went to 0.0014", keep rotating until it should have been 0" again. Instead it read 0.0002". Rotate some more and 0.0014", keep going (in same direction) down to 0.0002". If I change direction or rotation, the Federal will drop to 0.0001, but not 0. (But if I keep rotating, I get 0.0014 and 0.0002) So is it sticking, or this is sort of what happens? (0 wasn't really zero, it was 0.1000") I put a drop of oil on the tip of the "ball" and the behavior stayed the same.

Don't know if this belongs here, or in the measurement forum. Either way, I'm a little puzzled. Can someone enlighten me?
 
With the indicator in touch with your rod in the chuck, grab the chuck and wiggle it around. You could have slop in bearings, or clearance between ram and drill press body. All can lead to non repeatability. Second point 3 thou runout on a drill press, CELEBRATE and get on with making chips.
Spindle on lathe, or mill sure you should expect better.
 
Near enough is good enough. Unless you need to do exceedingly fine work it quite good enough.
 
The reason for the exercise was me blowing a press fit. Maybe I didn't tighten the chuck correctly on the drill bits. All I know is the reamer did nothing, because the hole was already too large. Hole was supposed to be reamed to 0.1875, but was already at 0.191. (After drilling with a 0.185" drill). Honestly, the TIR is better than expected. Guess the drills are not true, or there's slop in the bearings. I'll try @Asm109's suggestion to see if there's more movement than a gentle rotation of the pulley provides.

If I can't drill decent holes, won't be any celebrating here...
 
And since drill bits don't drill perfect holes....it all makes sense. Out of curiosity, what size drill bit did you use and what size reamer?
 
Yes, I know they don't drill perfect holes... The plan was to use a 0.182, then a 0.185, followed by the reamer to 0.1875. One of the drills didn't look right - can't remember which one. Like the tip was fuzzy. That drill must of been bent, or I just plain didn't tighten the chuck correctly. Probably was tired or distracted. I'll look at the two bits again. It would be nice to replace the bad bit, so it doesn't happen again.
 
Bob Korves said:
Drill bits and drill presses are not precision machines. You can used them to remove bulk metal to an under size dimension and then finish the holes to final size with a different machine or method.
Click to expand...
Indeed, as I have found out the hard way! Hoping to fix that by changing the machine. And perhaps improve the tooling.
 
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